为什么 C99 抱怨存储大小? [英] Why does C99 complain about storage sizes?

问题描述

这是我在 Linux 上编译的一些代码:

This is some code I'm compiling on Linux:

#include <net/if.h>

int main() {
  struct ifreq ifr;
}

gcc test.c 没问题.

gcc -std=gnu99 test.c 没问题.

gcc -std=c99 test.c 失败并出现以下错误:

gcc -std=c99 test.c fails with the following error:

test.c: In function ‘main’:
test.c:4:16: error: storage size of ‘ifr’ isn’t known

C99不喜欢Linux中struct ifreq的定义有什么不同?

What's different about C99 that it doesn't like the definition of struct ifreq in Linux?

推荐答案

这是预处理和 GNU C 与 C99 的一系列后果.

It's a chain of consequences of preprocessing and GNU C vs C99.

首先，net/if.h:

1. net/if.h 包括 features.h
2. 稍后，它在 #ifdef __USE_MISC 块内定义 struct ifreq.

1. net/if.h includes features.h
2. Later on, it defines struct ifreq inside a #ifdef __USE_MISC block.

所以:

1. 什么是__USE_MISC?-- 这是 BSD 和 System V 共有的东西
2. 此时是否已定义?-- 我们需要在 features.h
中检查

1. What is __USE_MISC? -- it is stuff common to BSD and System V
2. Is it defined at this point? -- We need to check that out in features.h

现在，features.h:

1. 当您使用 --std=c99 时，GCC 默认定义 __STRICT_ANSI__(因为这就是 C99)
2. 在预处理 features.h 时，当 __STRICT_ANSI__ 开启时，BSD 和 System V 功能不会启动.即 __USE_MISC 是未定义.

1. When you use --std=c99 GCC by default defines __STRICT_ANSI__ (since thats what C99 is)
2. While preprocessing features.h, when __STRICT_ANSI__ is on, the BSD and System V features don't kick in. i.e. __USE_MISC is left undefined.

备份到net/if.h:预处理后struct ifreq甚至都不存在了！因此，关于存储大小的投诉.

Back up to net/if.h: struct ifreq does not even exist after preprocessing! Therefore, the complaint about storage size.

您可以通过以下方式了解整个故事:

You can catch the whole story by doing:

vimdiff <(cpp test.c --std=c99 -dD) <(cpp test.c --std=gnu99 -dD)

或以任何其他方式区分它们(如 diff --side-by-side)而不是 vimdiff.

or diff'ing them in any other way (like diff --side-by-side) instead of vimdiff.

如果您希望使用 -std=c99 进行干净编译，则必须考虑包含 _DEFAULT_SOURCE 功能测试宏(对于 glibc 版本 >= 2.19；对于较旧的 glibc 版本，请使用 _BSD_SOURCE 或 _SVID_SOURCE)，以便在 C99 提供的功能之上启用所需的功能.

If you want this to cleanly compile with -std=c99, you must consider the inclusion of the _DEFAULT_SOURCE feature test macro (for glibc versions >= 2.19; for older glibc versions, use either _BSD_SOURCE or _SVID_SOURCE) so that the required functionality is enabled on top of what is offered by C99.

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